1. Technical Field
The present invention relates generally to an injection-locked frequency divider, and more particularly to an injection-locked frequency divider that is capable of adjusting oscillation frequency for itself.
2. Description of the Related Art
Individual countries are enforcing spectrum policies that strictly stipulate usages and licenses for respective frequency bands. In most non-European countries, the super-high frequency band, such as the 60 GHz band, is a band that can be used without permission or a license. Recently, broadband high-speed wireless communication systems have been researched to provide personal area wireless communication service that does not require such permission.
In order to implement an analog communication chip operating in a super-high frequency band, a phase-locked loop (PLL) or frequency synthesizer that operates at a super-high frequency should be ensured first.
In particular, a PLL, a frequency generator or the like has a phase detector in a loop in order to perform accurate frequency and phase locking. The phase detector may operate properly only if the frequency of an oscillation signal is reduced to a frequency that is suitable for the operating range of a common phase detector. Accordingly, there is required a frequency divider that receives the frequency of an oscillation signal in a super-high frequency band and that operates stably in a broad operating range, i.e., a broad oscillation range, in which the frequency can be reduced to a frequency that is suitable for the operating range of the phase detector.
Although various types of frequency dividers have been proposed, an injection-locked frequency divider (ILFD) has the best characteristics in terms of the dynamic range of operation frequencies because the ILFD can operate in a tens of GHz band. In particular, a ring oscillator-based ILFD has a significantly smaller circuit area and a larger successfully synchronizable range than an LC oscillation circuit-based ILFD.
An injection locking phenomenon is a phenomenon that is generated by a nonlinear oscillation circuit when a signal having a different frequency is injected into a nonlinear oscillation circuit having its own oscillation frequency from the outside. The oscillation circuit into which the different frequency has been injected basically oscillates at an oscillation frequency, but a beat corresponding to the difference between the injection frequency and the oscillation frequency appears along with the waveform of the oscillation frequency. Meanwhile, when the difference between the oscillation frequency and the injection frequency is reduced to a value smaller than a specific threshold value, the beat disappears and a phenomenon in which the oscillation circuit oscillates at the injection frequency instead of its own oscillation frequency occurs. This phenomenon is called injection synchronization.
An injection locking phenomenon also occurs when oscillation frequency is a harmonic (that is, n times when n is a positive integer) or sub-harmonic (i.e., 1/n times) of the injection frequency. Using an injection locking phenomenon that occurs at a sub-harmonic, an injection-locked frequency divider that frequency-divides an injection frequency by 1/n can be constructed.
Since the injection-locked frequency divider is intended to operate in the state of being locked to the sub-harmonic frequency of the injection frequency, the desired sub-harmonic frequency for the injection frequency and the oscillation frequency inside the oscillation circuit must be so close to each other as to generate the injection locking phenomenon. Accordingly, the stability of the oscillation frequency of the oscillation circuit is important.